Single crystal silicon, which is the starting material in most processes for fabricating semiconductor electronic components, is commonly prepared according to the so-called Czochralski process. In this process, polycrystalline silicon, or polysilicon, is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon, and a single crystal ingot is grown by relatively slow extraction. After formation of a neck is complete, decreasing the pulling rate and/or the melt temperature enlarges the diameter of the crystal until a desired or target diameter is reached. The generally cylindrical main body of the crystal, which has an approximately constant diameter, is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter is gradually reduced to form an end-cone. Typically, increasing the crystal pull rate and heat supplied to the crucible forms the end-cone. When the diameter becomes small enough, the crystal is then separated from the melt.
As in known in the art, molten silicon (at about 1420 degrees Celsius (° C.)) will dissolve the surface of a silica (SiO2) crucible containing the melt. Some of the dissolved silica evaporates from the surface of the melt as SiO (silicon monoxide) while some of the dissolved silica becomes incorporated into the growing crystal. The remainder of the dissolved silica remains in the melt. In this manner, the crucible containing the silicon melt acts as a source of oxygen that is found in silicon crystals grown by the conventional Czochralski technique.
Oxygen in the silicon crystal may have both favorable and unfavorable effects. In the various heat treatment processes during the manufacture of various electrical devices, the oxygen in the crystal may cause crystal defects such as precipitates, dislocation loops, and stacking faults or it may cause electrically active defects resulting in devices with inferior performance characteristics. The solid solution of oxygen in the crystal, however, increases the mechanical strength of silicon wafers, and the crystal defects may improve the yield of conforming products by entrapping contaminants of heavy metals. Accordingly, oxygen content of the silicon crystal is an important factor for product quality that should be carefully controlled in accordance with the ultimate application for the silicon wafers.
The oxygen concentration in a conventional silicon crystal grown under Czochralski conditions prevalent in the industry varies along the length of the crystal. For example, the concentration is typically higher at the seed end than in the middle and/or at the bottom or tang end of the crystal. In addition, oxygen concentration typically varies along the radius of a cross-sectional slice of the crystal.
To address this oxygen control problem, attention has been given to the use of magnetic fields to stabilize convective flows in metal and semiconductor melts for controlling oxygen concentration and radial distribution to remove dopant striation, etc. For example, Lorentz forces, which can be generated in a conductive melt as a function of an induced current and an applied magnetic field may be used to dampen natural convective flow and turbulence. Convective flow or convection refers to the process of heat transfer in a liquid by the movement of the liquid itself.
In general, there are two types of convection: natural convention and forced convection. Natural convection occurs when the movement of the melt is due, for example, to density gradients arising from the presence of heaters. Forced convection occurs when the movement of the melt is due to an external agent such as rotation of the crucible and/or crystal. In the normal Cz process, the melt flow is controlled by the motion of the crucible and the crystal being grown, and by the heat flow in the system. Because the melt is at a high temperature (>1412 C) and there can be large heat fluxes, the temperature gradients in the melt can be large, so thermal convection plays a large role in determining the melt flow, Melt flow in an axisymmetric crystal puller can be described using the components of cylindrical coordinate system (e.g., r, θ, z). For example, forced convection resulting from rotating a crucible 3 generally produces movement of the melt azimuthally in the θ direction (see FIG. 1A), and natural convection generally produces a global thermal convective roll in which melt moves radially in the r direction and vertically in the z direction (see FIG. 1B). As known in the art, and as described in more detail below, the direction of movement of the conductive liquid (i.e., melt) and the shape of the magnetic field being applied to the melt determine the direction of an electric field and/or electric current that will be induced in the melt. When an electric charge (e.g., current) moves in a magnetic field a force will act on that moving charge (i.e., Lenz's Law). In view of these principles, various magnetic field configurations have been used to generate forces in a silicon melt to stabilize convective flows, control oxygen concentration, and to remove dopant striation, etc during a crystal growing process.
There are three conventional types of magnetic field configurations used to stabilize convective flows in conductive melts, namely, axial, horizontal, and cusped.
The axial (or vertical) magnetic field configuration (e.g., see FIG. 2A) has a magnetic field parallel to the crystal-growth direction. In this configuration, movement of the melt in the θ direction induces an electric field in the r direction, but minimal if any current can flow in the melt. However, movement of the melt in the r-z plane as illustrated in FIG. 1B induces currents in the θ direction that flow counterclockwise near the top of the melt and clockwise near the bottom of the melt (e.g., see FIG. 2B). Please note that an “X” appearing on the right side of crucible and a corresponding “.” appearing on the left side indicates a counter clockwise direction as viewed from the top of crucible, and an “X” appearing on the left side of crucible and a corresponding “.” appearing on the right side indicates a clockwise direction as viewed from the top of the crucible. Since minimal current is induced as a result of movement of the melt in the θ direction, there are minimal, if any, forces produced in the melt to retard melt flow in the θ direction. However, the current induced in the melt as a result of convective flow in the r-z plane produces forces in the melt that retard the melt flow which produced it.
In the horizontal (or transverse) magnetic field configuration (see FIG. 3), two magnetic poles (not shown) are placed in opposition to generate a magnetic field perpendicular to the crystal-growth direction. The horizontal configuration has the advantage of efficiency in damping a convective flow at the melt surface. But its non-uniformity both axially and radially and the complex and bulky setup introduce additional design consideration when applying the horizontal magnetic field configuration in large diameter Czochralski growth processes. In this configuration, the retarding forces are not axisymmetric, so the azimuthal symmetry of the system will be lost.
The cusped magnetic field configuration (e.g., see FIG. 4A) provides some advantages over of the axial and horizontal magnetic field configurations. A pair of coils (not shown) placed coaxially above and below a melt-solid interface and operated in an opposed current mode generates a magnetic field that has a purely radial field component near the melt surface and a purely axial field component near the center of the melt. In this manner, the cusped magnetic field configuration attempts to preserve the azimuthal symmetry at the interface between the melt and the crystal. As used herein, azimuthal symmetry refers to a property having the same values independent of azimuthal position, while having different values at different radial positions. In this configuration, movement of the melt in the θ direction induces an electric field that causes current to flow downward at the edge of the melt and upward in the center (see FIG. 4B), and movement of the melt in the r-z plane as illustrated in FIG. 1B induces currents in the θ direction that flow in counterclockwise direction near the top of the melt and a clockwise direction near the bottom of the melt (see FIG. 4C). The currents induced in the melt as a result of melt flow in the θ direction and r-z plane each produce forces in the melt. The forces produced by the induced electrical currents retard the melt flow which produced the respective currents.
As these conventional magnetic fields are generally limited to retarding melt flow, an improved control of the crystal growth process is desired to address the inability of these conventional magnetic field configurations to selectively generate forces in the melt to accelerate melt flow.